Answer – The Correct option is (B) 20cm.
Focal length of concave mirror is –
1/f = 1/v + 1/u
Here, according to the question
v=-30cm
Magnifying factor (m) = m/u = -y/x
m = -5/10
Since m = v/u
-5/10 = v/u
-5/10 = (-30)/u
∴ u = 60cm
Also, the object distance is always negative,
Therefore u= -60cm.
Applying the formula,
(1/ v) + (1/u) = 1/f
1/f=1/-60 + 1/-30
1/f= -1/60 – 1/30
1/f=-90/1800
f = – 20 cm
Since the focal length is in front of the mirror, its sign is negative.